While the invention is applicable to most engines employing electrical pulses for firing spark plugs, as well as other engines where transducers detect cylinder ignition in the absence of an electrical signal, such as in the case of diesel engines, never-the-less it will be explained in respect to its application to both single and multiple engine aircraft.
The aircraft tachometer is in most cases, the only indication of engine performance in the cockpit. Loss of an operational tachometer can severely impair the pilot's ability to operate the aircraft safely. Prior art aircraft tachometers exibit several undesireable qualities:
They may lack sufficient display precision to supply diagnostic information to the pilot during pre-flight check out;
They provide little other diagnostic information to the pilot;
They are mechanical devices, mechanically connected to the aircraft's engines;
They exhibit linearity error as RPM increases over the operating range of the device;
They are prone to mechanical aging and failure;
They provide no alternate or redundant source of RPM in case of failure of the cable or connecting assemblies.
These older mechanical tachometers typically employ a rotating cable, within a cable housing, that connects the engine to the tachometer. Failure of the cable, its oil seals, gearing, or housing can be catastrophic to the pilot as well as expensive to fix or repair.
An electronic tachometer is able to overcome all of the above mentioned problems, delivering its RPM information in a more accurate numeric display instead of the traditional analog dial-type display.
In addition, an electronic tachometer can provide the pilot additional information regarding the operation of his aircraft more reliably and at a lower overall cost.
Pilots flying twin engine aircraft have an additional need for an electronic tachometer. The tachometer can calculate RPM information from both engines simultaneously and inform the PILOT of the difference so that adjustments can be made to exactly match the two engine's RPM.
Additionally, aircraft applications require at least three significant digits of RPM to be displayed verses the normal two significant digits found on automotive digital tachometers. The problem of achieving this additional accuracy was overcome by devising a method and apparatus to minimize the errors in sampling the firing pulses received from the engine's magnetos.
Typical electronic tachometers will simply count the number of firing pulses received during a fixed period of time, usually 1/3 to 1/4 second to keep the display frequently updated. This method cannot account for the time from the beginning of the sampling period until the first counted firing pulse and the time to the end of the sampling period from the last counted firing pulse. These two time periods represent un-determined fractions of firing pulse periods that cause significant error at lower RPM operation.
An alternate method is to measure the duration of one firing period between consecutive firing pulses. Knowing the units of time this measurement is made in and the number of cylinders in the engine will allow calculation of RPM. The method is simple to implement in a microprocessor but fails in accuracy at higher RPM unless an extremely accurate timer is available to measure the firing period. An example is that as little as 1.2 microseconds need be measured to determine between 5000 and 4999 RPM. This method is not practical for low cost applications and its accuracy will also suffer from slight external mechanical variations, such as engine or magneto wear, bearing wobble, cylinder compression differences, fouled plugs or bad ignition wires.
The solution is to count cylinder firings for at least a pre-determined amount of time that is synchronized with the first pulse counted and optionally extended to the next pulse immediately following (or co-incident with) the end of the pre-determined time. This yields an integer number of firing pulses counted during the sample period, eliminating the error of the first method, and does not require a high precision timer. The timers currently existing within the selected microprocessor will suffice for this job.